1. Field of the Invention
This invention relates in general to drive circuits for nutating motors and, more particularly, to current control circuits for nutating motors.
2. Description of the Prior Art
A typical nutating motor has an electromagnetically stepped rotor which moves with a wobbling motion about the axis of an output shaft. The rotor is a magnetically permeable plate which has a ring gear portion which is in rolling contact with another ring gear fixed to a stator portion of the motor. The stator has a plurality of windings for sequentially energizing a series of electromagnets circumferentially spaced around the stator. The stator windings are sequentially activated causing the rotor to nutate or wobble around the axis of the output shaft. If the number of teeth on the ring gears are different, the rotor will move an angular distance equal to the difference in the number of teeth between the two gears for each nutation cycle.
A typical nutating motor and the drive circuits therefor are described in U.S. Pat. No. 3,492,515. A drive circuit for a nutating motor includes a distribution network which sequentially provides drive currents to the stator windings of the motor. Output signals from a ring counter operate power transistors connected to each of the windings and the speed of the motor is controlled by varying the output signal frequency. Some motors have a pair of adjacent windings energized. This insures that at least one winding is always energized to maintain continuous control of the rotor. Another prior art control circuit for a nutating motor operates from three phase AC signals which are rectified by a diode network to provide a sequence of output current pulses for each winding of a motor. Many installations using nutating motors do not have three-phase AC power available. Those installations which do have three-phase AC power available often do not have means for varying the frequency of the signals so that the speed of a nutating motor is not adjustable.
Some control systems for nutating motors permit the motor to be operated over a range of speeds. These systems do not compensate for low speed and high speed motor operation. At high speeds, fast current rise and fall times are required to operate the motor so that the stator magnetic circuits quickly saturate and discharge to provide precise control for the motor. At low speeds, the currents quickly saturate the winding and continue to rise to levels which are far in excess of the current levels required to hold the rotor in position. These excessive current levels cause undesirable heating of the nutating motors. Limiting the current levels, such as by inserting resistance in the stator winding circuits, adversely affects the current rise and fall times. Regulating the current through the motor winding with a constant current regulator causes excessive power to be dissipated by the current regulator. The use of switching circuits is complicated by the reverse emf generated when a switching circuit abruptly opens the current path through the winding. The relatively high voltages generated under such circumstances are normally shunted by a normally back-biased quenching diode connected across the winding. However, such diodes limit the reverse emf to less than a volt. Consequently, the current decay through the winding is very slow, thus limiting the transient response of the system. While zener quenching diodes would permit a larger reverse emf, and hence a faster current decay, the power dissipated in the zener diode would be greatly increased.